Engines such as diesel or other lean burning engines generally provide more complete fuel combustion and better fuel efficiency than other types of engines. While these engines can be very efficient, they generally operate at higher temperatures and pressures than comparable non-lean burning engines. With the higher pressures and temperatures, oxides of nitrogen (NOx) emissions including nitric oxide (NO) and nitrogen dioxide (NO2) are typically higher as oxygen and nitrogen tend to combine more easily at higher temperatures. However, such NOx emissions have been known to cause environmental issues and thus are subject to emissions control regulations. These emissions control regulations limit the amount of NOx emissions engines are allowed to emit during normal operation and have resulted in the widespread use of NOx reduction devices in engine exhaust systems in order to reduce the NOx emissions to the required levels.
Specifically, one such after treatment system that has been widely used is known as a selective catalytic reduction (SCR) system. SCR systems generally utilize a catalyst that converts NOx gases into nitrogen gases and water with the aid of a reducing agent. The reducing agent typically contains hydrogen or the like, which is capable of removing oxygen from NOx gases. Commonly used reducing agents are ammonia, Diesel Exhaust Fluid (DEF), urea, hydrocarbon-containing compounds and the like. The introduction of the reducing agent to the after treatment system allows for it to be adsorbed onto the catalyst to facilitate the reduction process. Typically, a solution of the reducing agent is internally or externally carried by an engine, and a supplying system injects the reducing agent into the exhaust gas stream entering the SCR system.
During engine operations, the unburned hydrocarbons in the exhaust stream enter the SCR system and can adsorb onto the catalyst. The hydrocarbons can be in liquid phase or can condense into the liquid phase upon contacting the catalyst surface. Once in the liquid phase, the hydrocarbons can adsorb and accumulate on the catalyst pores and void volumes. Unburned hydrocarbons are particularly known to be produced in the engine exhaust during pro-longed periods of engine idle usage and/or low temperature operations. If such a situation is followed by relatively rapid heating of the catalyst, the hydrocarbons can ignite and cause an exothermic event that could potentially damage the catalyst. Alternatively, if the accumulated hydrocarbons don't ignite, they can inhibit the catalyst performance by blocking the active catalyst sites used for oxidation of hydrocarbons and carbon monoxide (diesel oxidation catalyst) and conversion of NOx gases into nitrogen gases and water (selective catalytic reduction).
For this reason, it is desirable to be able to determine the amount of accumulated hydrocarbons that may be trapped in an exhaust after treatment system on a real-time basis so that when levels reach a predetermined level, the issues may be dealt with so that the hyrdocarbons may be released from the after treatment safely and efficiently. Some examples of methods for releasing hydrocarbons include, but are not limited to, manipulating operating/idle conditions, modification of engine calibration/mapping, and limiting engine power output/temperature.
While it would be desired to directly measure the accumulated hydrocarbon level in the after treatment system itself, this can be a difficult characteristic to measure directly. However, it has been found that an accurate model can be utilized to calculate with some accuracy the amount of hydrocarbon build-up. One model that has been used to perform this estimation is Vanadia SCR HC Accumuation Model. One of the inputs needed to utilize this model is the temperature of the exhaust as it enters, or at various points in, the exhaust after treatment system. However, some engine systems do not incorporate a thermocouple or direct measuring sensor located at the points wherein this temperature is needed for the corresponding hydrocarbon accumulation model being used. Accordingly, in order to utilize the aforementioned model (or for any other purpose), it is desirable to have a method for calculating an exhaust temperature entering, or at various points within, the after treatment system utilizing available inputs other than directly measured temperatures.
It is known to calculate engine exhaust temperatures through indirect means, i.e. directly measured and/or calculated inputs other than the temperature itself. For example, U.S. Pat. No. 8,205,606 issued on Jun. 26, 2012 to Rodriguez et al. entitled “Model for inferring temperature of exhaust gas at an exhaust manifold using temperature measured at entrance of a diesel oxidation catalyst” (the '606 patent) discloses one such method. As the title suggests, the '606 patent discloses a method for calculating the temperature of an exhaust gas at an exhaust manifold based upon the temperature measured at the entrance of the exhaust after treatment system. More specifically, the '606 patent discloses calculating this exhaust temperature based upon related parameters including engine operation conditions, ambient conditions, exhaust system characteristics, engine speed and load, etc. However, the '606 patent does not disclose a method for calculating an exhaust temperature at an exhaust after treatment system inlet or at various points therein.
Accordingly, there is a need for a method for calculating an exhaust temperature at the exhaust after treatment inlet or at various points therein utilizing available inputs other than a directly measured temperature.